1. Field of the Invention
This invention generally relates to a method for producing a hollow plastic product and the resulting structure, and, in particular, to a method for producing a hollow plastic product of any desired shape having a radially extending portion, such as a flange or bracket, and the resulting structure. A hollow plastic product resulting from the present method includes various ducts, pipes and tubes for use in automobiles, in particular in engine rooms thereof, and for use in various home appliance and industrial goods for flowing gas and liquid, such as water and oil.
2. Description of the Prior Art
Typically, a blow molding method is used for producing hollow plastic ducts, pipes and tubes. However, use may also be made of an injection method or an extrusion method for that purpose. The blow molding method is most advantageous in producing a hollow plastic product since the degree of freedom is very high in designing such a hollow plastic product. However, if it is desired to produce a hollow plastic product including an integrated radially extending portion, such as a flange or bracket, the blow molding method is not so easy to use.
For example, if a hollow plastic product having a flange at its end as shown in FIG. 1 were to be produced by the blow molding method from the same parison in one step, the resulting structure would become as illustrated in FIG. 2. That is, in FIG. 2, the shape of an intended flange is indicated by the dotted line 3. However, since the parison 1 is inflated by the pressurized gas supplied therein, that portion of the parison 1 that is located in a flange defining cavity section of a mold 2 is expanded radially so that an undesirably thin portion will be formed at a corner 4, and the resulting flange is undesirably thinner than intended. Thus, it is almost impossible to form a radially extending portion, such as a flange or bracket, integrally with a tubular portion at the same time by the blow molding method.
Under the circumstances, it is conceivable to use an insert blow molding method in which a radially extending portion, such as a flange or bracket, is formed by an injection method in advance, and, then, such a prefabricated element is used as an insert element in the blow molding method. That is, such an insert element is inserted into a corresponding recess defined in a mold cavity before applying a parison into the mold cavity. And, such an insert element is integrated in the resulting blow molded plastic product when a pressurized gas is injected into the parison. However, in this method, since difficulty may be encountered in placing an insert element, there is a limit in the degree of freedom in designing, and, moreover, the joint between the insert element and the blow-molded portion is not very strong and thus rather unreliable.
Another approach would be to use a post-welding method in which both of a radially extending portion, such as a flange or bracket, and a tubular portion are fabricated separately in advance, and, then, these separate elements are combined together by welding, for example by fusing them together. In this case, however, the process tends to be complicated and the joint between the radially extending portion and the tubular portion is rather unreliable.
The injection molding method can be advantageously used to form a flange. However, the injection molding method is not easy to use in producing a tubular portion of a hollow plastic product. When a tubular portion is to be produced by the injection molding method, use is typically made of a sliding core. As a result, the shape of such a tubular portion is necessarily limited to a one dimensional or straight shape and a tubular portion whose center axis varies two or three dimensionally cannot be produced.
In order to cope with this situation, there has been proposed a melting core method which allows to produce a hollow plastic product having an arbitrarily shaped tubular portion and/or a radially extending portion, such as a flange or bracket. In accordance with this melting core method, a meltable core made of an alloy having a low melting point commensurate in shape with a desired tubular portion is set in an injection mold cavity and a molten plastic material is injected into the cavity. Thereafter, heat is applied to have the meltable core melted away to thereby leave the molded tubular portion. In this manner, according to this melting core method, a hollow plastic product of any desired shape and/or having a radially extending portion can be manufactured. However, since use must be made of a metal alloy which can be melted at a relatively low temperature, there are various problems associated with this method.
Furthermore, the extrusion method is suitable for producing straight hoses and tubes, but this method cannot be used in producing a hollow plastic product having a radially extending portion, such as a flange or bracket.